Fixed KRuntime to build on mono/Linux

A Lot of changes were required to the build script, but most of the changes are just addition of if='IsMono' and if='!IsMono'. Highlights of some changes,I have to add Linux directory to the nuspec folder, because the current .nuspec files have file paths that follow windows file system structure. so i rewrite nuspec package for mono-kre and placed it  in the nuspec\Linux folder and direct build script to build nuget package from that if we are on Linux/mono and use the windows versions if we are on windows. Their is only one change in the source code which was a trivial one. I am here to answer any query about the changes i have a reason for every change.Personally tested it on both windows and Linux.